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<html lang="zh-CN">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>value=undefined</title>
</head>

<body>
    <script>
        //在计算机程序中，经常会声明无值的变量。未使用值来声明的变量，其值实际上是 undefined。
        var carname;
        console.log(carname);   //undefined
    </script>
</body>

</html>